Generally, optoelectronic oscillators (OEO) receive pump continuous energy from an optical source, such as a laser, in addition to energy in the form of direct current (DC) power from a power supply. The energy is converted into radio frequency (RF) and microwave signals based on efficient power conversion by meeting the required gain and phase characteristics at an oscillation frequency using a narrowband filtering mechanism. The OEOs typically experience a low loss optical delay, low temperature sensitivity, and can be realized in modular structures. These benefits often lead to a high quality factor and greater stability for the optoelectronic oscillators in both the short term and the long term, as compared to electronic oscillators.
For example, a self-injection locked phase lock loop (ILPLL) OEO is reported in U.S. patent application Ser. No. 13/760,767, the disclosure of which is hereby incorporated by reference herein in its entirety. In the OEO of that disclosure, a pure sinusoidal oscillating signal is sustained by actively filtering the sustaining signal from the surrounding interfering sources that contribute to close-in to carrier phase noise. Phase noise reduction is accomplished in several ways, including multiple lengths of optical delay lines and custom optical receivers to provide multiple electrical references for self-injection locking (IL), self-phase locked looping (PLL), and/or self-mode locking (ML) functions in a closed loop part. These components all contribute to a low noise stable RF oscillator configuration that supports self-sustained oscillations, provided that the electrical feedback signal fed to the modulator meets certain oscillation conditions in terms of its amplitude and phase. Meanwhile, corrections to any phase errors are provided using feedback systems to the tunable RF oscillator in self IL & PLL and optical laser source and modulator in self ML. In U.S. patent application Ser. No. 13/760,767, the frequency of the OEO output is controlled by several factors, such as the fiber delay length, the operating condition of an optical Mach-Zehnder modulator, and the band pass characteristics of an optical transversal RF filter used for narrowband filtering of the oscillating signal.
Such OEOs are conventionally assembled from discrete devices, require a large volume, have high power consumption, and require a great deal of real estate and cost. Additionally, due to the assembly of a large number of discrete parts, such OEOs are generally subject to vibration and G-sensitivity problems. These problems limit the utility of and possible applications for OEOs in several environments, such as both commercial and military environments.
Moreover, phase noise reduction using self-ILPLL does not achieve a sufficiently commercially viable product unless major low noise and efficient design innovations are considered to accommodate a low cost manufacturing process to meet the continuously increasing demand for quality services at a lower cost. As a result, there is a need for an optoelectronic oscillator having design topologies that meet the design constraints for implementation of a stable ultra-low phase noise frequency synthesizers in a small size and low cost.